This invention relates to the electronic cardiac pacer art and, more particularly, to a cardiac pacer including a controlled voltage multiplier for converting a low direct voltage into pulses of a relatively much larger amplitude and in a controlled manner.
Many electrical devices are powered by sources which operate at very low impedances, that is at low voltage and high current. Typical of such sources are common battery cells, which provide an output voltage of about 0.8 to 1.5 volts, human and animal nerve cells, thermoelectric, thermionic or betavoltaic generators which provide an output from about 0.1 to about 0.8 volts, as well as photocells, fuel cells and many others. Most common electronic devices or components require much higher operating voltages, for example vacuum tubes require 15 to 10,000 volts, transistors 2.5 to 250 volts and d.c. motors 1.5 to 10,000 volts.
Such an impedance mis-match is further illustrated by the specific example of an implantable artificial cardiac pacer, which requires about 2.5 volts for operation but which is powered from a rechargeable cell delivering about 0.8 volts or from a nuclear battery delivering from about 0.2 to about 0.75 volts. The low voltage of the source must be transformed into a higher voltage by a converter which is efficient, reliable and economical.
Many prior art voltage multipliers exist which operate according to the method of charging a plurality of capacitors in parallel with the available low voltage and then discharging the capacitors in series to provide a greatly increased voltage. In particular, such arrangements often employ a ladder network to isolate the power source from the high voltage discharge, and as the capacitors in the network become charged the voltage also rises across spark gaps connected in series with the capacitors. When the voltages on the gaps become sufficient, they break down placing all the capacitors in series across the load.
Such prior art arrangements are limited to relatively high operating voltages since the spark gaps in reality are voltage-actuated switches. While neon bulbs or four layer diodes might be substituted for the gaps to reduce the operating voltage to the range of tens to hundreds of volts, such arrangements nevertheless cannot be employed where it is desired to multiply fractional voltages. In addition, such arrangements have the d.c. power supply voltage appearing across the load, even during quiescent periods, and often in such arrangements the capacitors are not equally charged. Moreover, the operational requirements of an artificial cardiac pacer often render it desirable that the power supply deliver a pulsating signal having zero net average current and that all sections of the pacemaker circuit are operating at the same reference or ground potential. Voltage multipliers heretofore available, on the other hand, do not have the capability of delivering an output signal of zero net average current and, in most instances, the output thereof must float off ground.